Active electronically scanned array antenna for hemispherical scan coverage

ABSTRACT

An antenna architecture for hemispherically-scanning active electronically scanned arrays (AESA). The antenna architecture utilizes variable diameter disks of antenna elements configured in a conical implementation. The antenna elements are oriented such that the element boresight is normal to the surface of the conical structure. Beamforming takes place on each disk first, and them separately in combining the signals from each disk, thereby reducing complexity. The antenna optionally utilizes disks of antenna elements of the same diameter to form a cylindrical antenna, which when combined with a conical configuration create enhanced sectors while maintaining a hemispherical coverage capability. Further, use of two conical configurations can produce a fully spherical coverage capability.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of and claims priority to U.S.Non-Provisional application Ser. No. 12/955,374 filed on Nov. 29, 2010,now allowed, which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an active electronically scanned arrayantenna, and, more specifically, to an active electronically scannedarray antenna for hemispherical scan coverage.

2. Description of the Related Art

Radar systems use antennas to transmit and receive electromagnetic(“EM”) signals in various ranges of the EM band. While traditional radarsystems used moving parts to physically point the antenna towardsdifferent target fields, modern radar systems use a passiveelectronically scanned array (“PESA”) in which a central EM signal issplit into hundreds or thousands of paths by phase shift modules whichsend the signal into individual antenna elements (i.e. the antenna'selectrical conductor material). A single radar unit can containthousands of individual transmit receive modules (“TR”) rather than thesingle TR module of traditional radars, with each module functioning asan individual radar. Since transmission of the EM signal can beselectively delayed at each individual TR module, the electromagneticsignal, also called the “beam,” is steered without requiring movement ofthe antenna elements. In most radars, the TR module contains a receiver,power amplifier, a digitally controlled phase/delay element, and a gainelement.

In an active electronically scanned array (“AESA”) each antenna elementpossesses its own EM signal source. As a result, each individual AESAantenna element can transmit a different EM frequency and the radar cancapture a much more coherent radar profile of the target field. An AESAradar can steer the EM signal very quickly, and the TR modules canfunction in series to process a single project or function in parallelto complete several projects simultaneously. There are many additionaladvantages of AESA radars that can be found in the literature.

Despite these advantages, there are still significant obstacles towidespread adoption of AESA-based radar systems. For example, an AESAradar system using hundreds or thousands of TR modules can beprohibitively expensive.

BRIEF SUMMARY OF THE INVENTION

It is therefore a principal object and advantage of the presentinvention to provide a hemispheric ally-scanning AESA digital antenna.

It is another object and advantage of the present invention to provide acombined cylindrical/conical antenna architecture for ahemispherically-scanning AESA radar.

It is yet another object and advantage of the present invention toprovide a hemispherically-scanning AESA that does not require individualchannels for each individual element.

Other objects and advantages of the present invention will in part beobvious, and in part appear hereinafter.

In accordance with the foregoing objects and advantages, the presentinvention provides a combined cylindrical/conical antenna architecturethat significantly reduces the number of channels from one for eachelement to one for each disk level.

According to a first aspect of the present invention is ahemispherically-scanning AESA architecture. The antenna comprises: (i) afirst lower region which is generally cylindrical and which is made upof a plurality of platters with antenna elements arranged on theexterior circumference of each of the platters; (ii) a first upperregion which is generally conical and which is also made up of aplurality of platters with antenna elements arranged on the exteriorcircumference of each of the platters; and (iii) one or moreamplitude/phase modules on each platter, where each amplitude/phasemodule is coupled to two or more antenna elements. According to oneembodiment of the present invention the platters are generally circular,and are stacked one upon another to form either the cylindrical array orthe conical array. To form the conical array, each of the stackedplatters in the conical region have a diameter which is smaller than thediameter of the platter beneath it in the stack.

A second aspect of the present invention provides a beamforming networkin which each amplitude/phase module comprises a sum (Σ) azimuth beampath and a delta (Δ) azimuth beam path, and where the sum (Σ) azimuthbeam paths and the delta (Δ) azimuth beam paths from each individualplatter are combined.

A third aspect of the present invention provides a method for radartarget detection. The method includes the steps of: (i) providing anantenna with a plurality of antenna elements arranged on the exteriorcircumference of a plurality of platters, and a plurality ofamplitude/phase modules, where each of the plurality of amplitude/phasemodules is coupled to two or more of the antenna elements; (ii)selecting a first subset of the plurality of antenna elements, where thesubset ranges from one antenna element to every antenna element in theantenna; (iii) receiving a signal; (iv) calculating a sum (Σ) azimuthbeam and a delta (Δ) azimuth beam for each amplitude/phase module whichis coupled to an antenna element in the subset of selected elements(ranging from one to all elements); (v) combining each sum (Σ) azimuthbeam and a delta (Δ) azimuth beam from every amplitude/phase module oneach platter into a single sum (Σ) azimuth beam and a single delta (Δ)azimuth beam for that platter; and (vi) forming an elevation beam.According to one embodiment of the present invention the methodoptionally includes one or more of the following steps: (i) convertingeach of the single sum (Σ) azimuth beam and the single delta (Δ) azimuthbeam to a digital signal prior to forming the elevation beam; (ii)downconverting the calculated sum (Σ) azimuth beam and the calculateddelta (Δ) azimuth beam; (iii) demodulating the digital signal; and/or(iv) amplifying the received signal.

A fourth aspect of the present invention provides radar system with: (i)an antenna having a plurality of antenna elements arranged on theexterior circumference of a plurality of platters, and a plurality ofamplitude/phase modules, where each of the plurality of amplitude/phasemodules is coupled to two or more antenna elements; (ii) means forselecting a first subset of the antenna elements; (iii) means forreceiving a signal; (iv) means for calculating a sum (Σ) azimuth beamand a delta (Δ) azimuth beam for each amplitude/phase module which iscoupled to an antenna element in the selected subset; (v) means forcombining each sum (Σ) azimuth beam and a delta (Δ) azimuth beam fromevery amplitude/phase module on each platter into a single sum (Σ)azimuth beam and a single delta (Δ) azimuth beam for that platter; and(vi) means for forming an elevation beam. According to one embodiment ofthe present invention the system optionally includes one or more of thefollowing: (i) means for converting each of the sum (Σ) azimuth beamsand the single delta (Δ) azimuth beams to a digital signal prior toforming the elevation beam; (ii) means for downconverting the calculatedsum (Σ) azimuth beam and the calculated delta (Δ) azimuth beam; (iii)means for demodulating the digital signal; and (iv) means for amplifyingthe received signal.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The present invention will be more fully understood and appreciated byreading the following Detailed Description in conjunction with theaccompanying drawings, in which:

FIG. 1 is a schematic representation of a combined cylindrical/conicalAESA according to one embodiment of the present invention;

FIG. 2 is a schematic representation of a single circular plattersection of the AESA radar according to one embodiment of the presentinvention;

FIG. 3 is a representation of a simplified four-disk antenna where eachconcentric disk has a progressively smaller diameter;

FIG. 4 is a top view of the four-disk antenna depicting the orientationof the elements according to one embodiment of the present invention;

FIG. 5 is a flowchart showing beamforming according to one embodiment ofthe present invention;

FIG. 6 is a flowchart showing beamforming according to one embodiment ofthe present invention;

FIG. 7 is a representation of an antenna according to one embodiment ofthe present invention;

FIG. 8 is a representation of an antenna with hemispheric coverageaccording to one embodiment of the present invention;

FIG. 9 is a representation of an antenna with hemispheric and horizoncoverage according to one embodiment of the present invention;

FIG. 10 is a representation of an antenna with spheric coverageaccording to one embodiment of the present invention; and

FIG. 11 is a schematic representation of the locations of elements in anantenna according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, wherein like reference numerals refer tolike parts throughout, there is seen in FIG. 1 a schematicrepresentation of a side view of a combined cylindrical/conical AESAradar denoted generally by numeral 10. The lower portion 18 of antenna10 is comprised of a generally cylindrical array of individual antennaelements, wherein the elements are disposed at the exteriorcircumference of the cylinder, as discussed in greater detail below. Anupper portion 20 of the radar is comprised of a generally conical arrayof individual antenna elements, wherein the elements are similarlydisposed at the exterior circumference of the cone.

FIG. 2 is a representation of a single circular disk 14 from antenna 10,constructed with radiating elements 12 along the edge of the structure.Disk 14 can be constructed with any element type and with anypolarization characteristics required or desired by the designer. Eachcircular disk 14 can optionally include, among many other things, apower amplifier, a circulator, a low noise amplifier (“LNA”), abuilt-in-test circuit, and component packaging devices, depending on thedesign requirements. Although the embodiment described herein containseach of these elements, it will be recognized by one or ordinary skillin the art that variations of the general design can be employed tosatisfy the specific needs of an end-user.

Associated with each disk 14 are one or more amplitude/phase (“amp/ph”)modules. Each amp/ph module services multiple antenna elements 12, andthe number of amp/ph modules in the radar or associated with each row ofthe radar will vary depending upon the number of antenna elements inthat row and the number of simultaneous active elements in a givenconfiguration. For example, if one-fourth of the antenna elements are tobe active at a given time, each amp/ph module will service fourelements. If one-third of the antenna elements are to be active at agiven time, each amp/ph module will service three elements. If allantenna elements are to be active at the same time, amp/ph module can beassociated with each element. However, when a single amp/ph moduleservices multiple elements, there is both a component reduction and acost savings.

Each amp/ph module contains two controlled paths, one corresponding to asum “Σ” beam adjustment (used on both transmit and receive) and onecorresponding to a delta “ΔA” Azimuth beam adjustment (used on receiveonly). The Σ Azimuth and Δ Azimuth paths from all amp/ph modules in asingle disk 14 are combined together. This is the transmit drivedistribution point for the disk, and is the combination point for the Σand Δ receive paths (which can be digital or analog).

By combining the cylindrical/conical antenna shape with the 3:1 elementselection in Azimuth, and each element set of each disk combined to forma set of azimuth beams, beamforming in the elevation dimension can becompletely accomplished through the combination of azimuth beams. Thecomplete reduction of one dimension at a time is just one advantage ofthis invention (compared to a scenario for which two dimensions ofelements must be combined at the same time).

The ability of a typical radar system to scan off antenna boresight istypically limited by the projection of aperture in the direction of avolume of interest, as well as the radiation pattern of a given element.In general, the projection of the antenna aperture, sometimes referredto as the “effective aperture,” is reduced by the cosine of the scanningangle multiplied by the aperture dimension corresponding to the scanneddirection. For example, scanning 60 degrees off boresight of a planarsurface will reduce the receive aperture by 50%. The radiation patternof a given element can vary depending on the element type, and thedimensions of the element relative to the wavelength of the frequency ofinterest. In general, the element pattern can be approximated as apolynomial multiplied by a function of the cosine of the scanning angle,often the cosine squared. This results in peak element gain in thedirection of boresight, and reduced gain off boresight.

In order to overcome these limitations, one embodiment uses a series ofconcentric “disks” of progressively smaller diameter to achieve anoverall tilt angle with respect to the horizon. FIG. 3, for example,shows an example of such an embodiment with four concentric disks 14 ofprogressively smaller diameter which create an effective tilt angle, θ,referenced to the horizon. This angle then becomes the angle ofreference for steering a beam. The steering angle from this reference,φ, can be added or subtracted from θ to determine the angle withreference to the horizon that can be achieved. As an example, if θ=45degrees, the array could achieve 0-90° elevation coverage by scanningonly φ=45 degrees off boresight, which maintains significant aperture.The actual tiltback angle can be chosen to maximize performance in agiven angular region of the surveillance volume. For example, if greatergain were required at the horizon, but the application required somecoverage to 90° elevation, the tiltback could be set to 30° so anaperture of 1 m would see a cos(30) =0.866 m effective aperture at thehorizon and a cos(60)=0.5 m efficient aperture at zenith.

One advantage of using circular disks is that any elements from any diskcan be selected for a given beam, which allows for essentially uniformangular coverage over 360 degrees in azimuth for any elevation position.In order to keep element spacing uniform on each disk, the number ofelements may decrease with each smaller circular disk. Shown in FIG. 4is a top view of four stacked disks 14 with progressively smallerdiameters, with radiating elements 12. According to another embodiment,the orientation of each of the individual disks 14 can be rotated toeasily achieve any number of different elemental lattice configurations.

In one embodiment, each disk 14 contains at least radiating elements 12,element selection circuitry, amplification, magnitude and phase control,and an azimuth beamforming network to combine elements coherently. Theelement selection circuitry is used to reduce the number of magnitudeand phase control components required. For instance, if one-third ofeach disk were used to form a beam, than a 3:1 switch could be used toroute elements 120° apart to a common set of amp/ph modules. Any othercomponents present on the disk depend on the implementation of theelevation beamforming network. In the digital case, shown in FIG. 5,after the azimuth beamformer has formed the sum (Σ) and Difference (Δ)beams for monopulse estimation, the data streams are downconverted fromRF to a lower frequency for analog to digital conversion. After analogto digital conversion and digital demodulation (not shown), thecontributions from the different disks are combined digitally to formthe elevation beam on receive.

On transmit, the process is reversed with digital coefficients convertedto analog at each disk, run through the azimuth beamformer, andtransmitted out of the elements. In the analog elevation beamformingcase, the analog outputs from each disk's azimuth beamformer are sent toan elevation beamformer prior to frequency conversion, sampling, andsignal processing, as seen in FIG. 6. This architecture allows forhemispheric coverage and the ability to scan to any location on thehemisphere while reducing the number of magnitude and phase controlcomponents and significantly reduced beamforming components and logiccompared to previous hemispherical scanning antenna implementations.

In yet another embodiment, disks 14 can be added to the antennastructure to enhance aperture (and hence gain) in any direction, withthe number and diameter of each disk defining an envelope of theradiating elements, as shown in FIG. 7. Alternatively, the antennaenvelope can be any shape that suitably contains the necessary elementsand satisfies other design requirements emanating from the anticipateduse(s) of the antenna. Indeed, several design parameters of an AESAradar's shape are variable according to the specific needs of the user.These include, among others, the following: (i) the height of thegenerally cylindrical portion of the radar (with a height of zero ifonly the conical portion is used); (ii) the diameter of the cylindricalportion, if used; (iii) the height of the generally conical portion ofthe radar; (iv) the diameter of the cone's bottom section; (v) thediameter of the cone's top section; (vi) the angle of the outer sectionof the generally conical portion; and (vii) the curvature of the conicalsection (which can be used for the purpose of coverage volumeoptimization).

In this manner, coverage can be obtained in a number of differentmanners, including hemispheric coverage (shown in FIG. 8), hemisphericwith extra energy at the horizon (FIG. 9), or even spherical coverage(FIG. 10). In the embodiment shown in FIG. 9, one implementation couldutilize all disks for 0° to 60° elevation coverage, and only utilize theconical section to form beams from 60° to 90° elevation.

Unique benefits of this architecture to obtain hemispheric coverageinclude:

-   -   1) Scalability and flexibility: By adding disks, power and gain        can be easily tailored in any search section of interest.    -   2) Uniform Azimuth coverage over 360° for a given elevation        angle: Multi-faceted planar arrays have been used to provide        360° azimuth coverage and may provide 90° of elevation coverage        if tilted properly. However, when steering off boresight in        azimuth, the active plane will have sensitivity and accuracy        degradation as the effective aperture of the planar surface is        reduced. In addition, multi-faceted arrays will typically        require more components to achieve the same coverage volume and        performance characteristics.    -   3) Ability to scan asynchronously: Unlike a mechanically scanned        planar array at a tilt-back angle that rotates, any azimuth or        elevation position can be serviced at any time. This makes the        architecture very flexible at adapting to dynamic operational        scenarios.    -   4) Reduction in Control Channels: Other antenna architectures        that provide hemispheric coverage such as a geodesic dome design        require complex and expensive receive channels. Each element        output must either be combined in a complicated switching        network or sampled directly to provide allow a beam to be formed        in any direction.    -   5) Graceful Degradation: By arranging the elements at the disk        level, if a single beamformer or analog to digital converter (in        the digital elevation beamforming case) were to fail, there        would not be significant degradation in either dimension.

According to one embodiment of the present invention, FIG. 11 showselement locations for an antenna having 20 disks, each with between 60and 30 elements per disk with an effective tilt-back angle of 34.5degrees for the conic section. This array, therefore, would have a totalof 1035 elements. Assuming one-third of the array (335 elements) isactive to form a coherent beam, the element locations shown in FIG. 11would be used.

In yet another embodiment, the cylindrical portion of antenna 10 is afixed diameter with a height equivalent to 32 rows of antenna elements.Each row contains 72 elements for a total of 2304 elements in thecylindrical section. The conical section has a height equivalent to 16rows with a total of 744 elements; 24 elements are arranged in the toprow and the remaining 720 elements form the remaining 15 rows of thecone. In this preferred embodiment, there are 2304 elements in thecylindrical section serviced by a total of 768 amp/ph modules (24 ineach row). There are 744 elements in the conical section serviced by atotal of 248 amp/ph modules (with a variable number per row). However,the Σ Azimuth and Δ Azimuth paths from all amp/ph modules in a row arecombined together, so there are only 48 channels for the 48 rows of AESAradar 10. As a result, the radar is only required to control these 48channels—rather than 3048 channels for each antenna element—to create asingle beam type over the hemisphere of the radar.

All units are in meters with dimensions determined using half wavelength(λ) spacing in azimuth and elevation at 3.3 GHz (S-Band). The resultingaperture at the horizon is 0.6145 m². The gain of a radar antenna can becalculated as:

${G = \frac{4{\pi \cdot A_{e}}}{\lambda^{2}}},$where G is the gain of the antenna and A_(e) is the effective apertureof the antenna. The effective aperture is the physical area times theaperture efficiency of the antenna. Assuming 70% efficiency for theexample antenna would yield a gain of 28.1 dBi. If the tapered portionwere not there (as in the case where 2 separate radars were used tocover the horizon and zenith), the aperture would be reduced to 0.3419 mand the gain to 25.6 dBi. The elevation beamwidth would also increase,leading to lower detection accuracy. This shows the advantage of havinga single integrated antenna where the smaller discs can be used toprovide greater elevation coverage and enhance performance on thehorizon. This antenna requires 20 azimuth beamformers (1 per disk) and 1elevation beamformer (analog or digital). The number of down-conversionchains would be 20 in the digital beamforming case or 1 in the analogbeamforming case.

Although the present invention has been described in connection with apreferred embodiment, it should be understood that modifications,alterations, and additions can be made to the invention withoutdeparting from the scope of the invention as defined by the claims.

What is claimed is:
 1. An AESA antenna architecture comprising: a firstregion comprising a first plurality of antenna elements arranged on theexterior circumference of a first plurality of platters, said plattersforming a generally cylindrical array; a second region comprising asecond plurality of antenna elements arranged on the exteriorcircumference of a second plurality of platters, said platters being thesame shape and being concentrically stacked to form a generally conicalarray, wherein the diameter of each of said second plurality of plattersgradually decreasing from a platter having a maximum diameter at thebase of said generally conical array to a platter having a minimumdiameter at the top of said generally conical array; and a plurality ofamplitude/phase modules, wherein each of said plurality ofamplitude/phase modules is coupled to at least two antenna elements. 2.The antenna architecture of claim 1, wherein each of said first andsecond plurality of platters is circular.
 3. The antenna architecture ofclaim 2, wherein said first plurality of platters are stacked to formsaid generally cylindrical array.
 4. The antenna architecture of claim1, further comprising a beamforming network.
 5. The antenna architectureof claim 1, wherein each amplitude/phase module comprises a sum (Σ)azimuth beam path and a delta (Δ) azimuth beam path.
 6. The antennaarchitecture of claim 5, wherein the sum (Σ) azimuth beam paths and thedelta (Δ) azimuth beam paths from each platter are combined.